1. Field of the Invention
This invention relates to an image pickup apparatus which enables light metering information of high accuracy to be obtained in a short time.
2. Description of the Prior Art
In the art of single lens reflex cameras using silver-halide film, it has already been known to utilize the reflected light from either the film surface or a diffraction grating in front of the film plane in the through-the-lens (TTL) light metering.
In the art of image pickup apparatus using image sensors, for example, electronic still cameras, on the other hand, a trend of opinion has so far prevailed that the reflected light from the image receiving surface of the sensor is of no valuable use, because the brightness information of an object to be photographed can be obtained from the output of the image sensor by using integrating means or the like.
To provide for such image pickup apparatus with a picture monitor in the form of an optical finder, however, though a half mirror is only required to use for splitting part of the entering light beam off to the finder, there is a drawback that the sensitivity of the image sensor sharply drops, and the finder also gives dim or uncomfortable viewing.
This problem will be overcome by substituting a quick return mirror for the half mirror.
But, this leads to produce an alternative problem in photographic situations where information representing that brightness which occurs at a time when the mirror starts to move upward is necessary to obtain just at that time from the image sensor, because it takes a certain period to read out all signal for each frame from the image sensor. For example, 1/60 sec. for NTSC system, or 1/50 sec. for PAL or SECAM system has passed form the above-identified time before the light metering information is obtained. To complete the derivation of an exposure value with high accuracy in far shorter time than these values of the period as, for example in flash photography where adjustment of the firing time of the flash unit must be accurately controlled by a stop signal, therefore, the use of the output of the image sensor as the light metering information could not provide assurance that the stop signal is produced in so much accurately timed relation to the start of firing.